1. Field of the Invention
The invention relates to electronic circuits, and more particularly, to filtering unwanted frequency components from a signal.
2. Description of the Prior Art
FIG. 1 shows a block diagram of a typical wireless transmitter 100. The transmitter 100 includes a quadrature modulator 102, an intermediate frequency (IF) local oscillator (LO) 104, a first filter 110, an up-converter 106, a radio frequency (RF) LO 108, and a second filter 112. The quadrature modulator 102 mixes the IF_LO signal (differentially represented as LO_I+ and LO_I−) with an incoming in-phase signal I and mixes a 90 degree phase delayed version of the IF_LO signal (differentially represented as LO_Q+ and LO_Q−) with an incoming quadrature-phase signal Q. The results of the two mixing operations are added and buffered to produce a differential IF signal (IF+, IF−). The IF signal (IF+, IF−) is filtered by the first filter 110 and is then input to the up-converter 106 where it is mixed with an RF_LO signal. The result is amplified and then filtered by the second filter 112 before being transmitted.
The reason the first and second filters 110, 112 are required is to filter out any unwanted spurious frequency components that would otherwise cause the wireless transmitter 100 to fail transmission mask requirements. These unwanted spurious frequency components are caused primarily from strong harmonics output by voltage controlled oscillators 114, 116 in the IF LO 104 and the RF LO 108, respectively. Because of an increasing demand by the general public for smaller and more ergonomic designs, telecommunication equipment (and particularly subscriber handset) manufacturers have sought higher levels of functional integration within their respective integrated circuit (IC) designs. However, as the implementations of the first and second filters 110, 112 require relatively large valued capacitors and inductors, it is difficult to move these filters into an IC, and as a result, off-chip filters, such as external ceramic filters, are usually utilized. As will be recognized by a person of ordinary skill in the art, a similar filtering problem is also present in the demodulation path of a wireless receiver.
FIG. 2 shows a frequency response 200 of a 3rd order low pass filter, which could be implemented on-chip to allow the quadrature modulator 102, the up-converter 106, the first filter 110, and possibly the second filter 112 to be implemented as a single IC. As shown in FIG. 2, there are several disadvantages of using a 3rd order low pass filter to allow the integration of the wireless transmitter 100 as a single IC. For example, a fundamental frequency f of the IF LO 104 or the RF LO 108 in FIG. 2 is the IF frequency (IF+, IF−) in the case of the first filter 110, or is the RF output signal RF_out in the case of the second filter 112, respectively. Because the fundamental frequency f is not located at dc (also known as baseband at substantially 0 Hz), the 3rd order low pass filter actually attenuates the fundamental frequency f. This attenuation is shown in FIG. 2 as a dB value ALOSS, which increases as f increases in frequency. Additionally, the 3rd order filter provides insufficient attenuation on strong VCO odd harmonics such as the third harmonic 3f, the fifth harmonic 5f, and the seventh harmonic 7f. Increasing the order of the filter will further attenuate these strong harmonics; however, the additional circuit complications, increased power consumption, and increased attenuation of the fundamental frequency prevent higher than 3rd order filters from being practical solutions for integration into an IC.